The Hrb genes, Hrb98DE and Hrb87F, encode Drosophila homologs of the vertebrate A/B group hnRNP proteins, a major class of nuclear proteins that bind to premessenger RNA. Biochemical studies of the vertebrate proteins have indicated that they may be involved in the regulation of splicing as well as in packaging nascent transcripts into ribonucleoprotein (hnRNP) complexes. The aim of our work is to use the genetic and molecular techniques available in Drosophila to analyze the functions of this class of proteins in the whole organism. A small deletion that removes the entire coding region of the Hrb87F gene has been generated. Mutant flies that completely lack the Hrb87F gene are fully viable, and the females are fertile, but the males are almost completely sterile. The lack of an effect on viability is unexpected, since Hrb87F transcripts are present throughout development, and the protein is probably a component of hnRNP complexes in most if not all cells. This result implies that other proteins may be able to compensate for the lack of the Hrb87F protein in somatic cells and in the female germ line. In the male germ line, however, such compensation either does not occur or is inadequate. Genetic experiments indicate that the Hrb98DE gene interacts with Hrb87F, and is also likely to be required for male fertility. The Drosophila Rb97D gene was identified based on its extensive sequence homology to the Hrb genes. Based on this homology, it is likely to be an RNA binding protein. Two protein isoforms that differ by six amino acids are produced by alternative splicing of Rb97D transcripts. Rb97D transcripts are expressed at moderate levels during early larval and pupal stages, and at low levels during the rest of development. Genetic analysis demonstrates that mutations in this gene also lead to male sterility. The involvement of these three related RNA binding proteins, Hrb87F, Hrb98DE, and Rb97D, in male fertility suggests that such proteins must have crucial roles in spermatogenesis. Experiments are in progress to determine whether these proteins are required for the processing or stability of some or all RNA transcripts in the testes.